This page lists some of the more common terms regarding telescopes, telescope building, and astrophotography.  I have tried to give a brief definition of the terms within the context they are used in these web pages.  There may be other or more correct definitions.


Keep it Simple (KIS). The best all around design strategy. The simpler a thing is, the better it generally works. This is because there is just less to go wrong.

Accessory Terms

Auto Focuser

A motorized focuser that adjusts the position of the eyepiece while reducing vibrations from manual focusing.

Cheshire Eyepiece

A collimation tube with a 45 degree reflector (white or silvered) that reflects light onto the primary mirror to allow adjustment of the primary mirror orientation.

Combination Tube

A sight tube with a cheshire type reflector in the peep hole end.


Counterweights are fixed or sliding weights added to counterbalance the weight of guide scopes, cameras, heavy eyepieces, etc. Without counterweights, the added torque from heavy accessories would cause motor and tracking problems.


Dew is the water that condenses out of the atmosphere onto surfaces with temeratures at or below the dew point.

Dew Heater

An electrical heater that prevents dewing of optical components. Dew heaters use resistance wire or resistors to heat mirrors, eyepieces, or other components. Dew heaters replace the heat lost to the open sky but do not drastically warm the telescope.

Dew Point

The temperature at which the first drops of water begin to condense out of the air. As a surface cools below the dew point, it becomes covered with dew.

Dew Shield

A dew shield is just a cylinder that fits around a telescope lens to reduce radiative heat losses to the open sky. A dew shield blocks heat from radiating to the open sky in the same way that parking next to a house on a cold night prevents frost on the car window facing the house; the house blocks heat radiation (losses) to the sky and slows the windshield cooling. Dew shields are simple solutions that can reduce dewing, but not completely eliminate dew problems-they only slow the heat loss.

Nichrome Wire

A nickel chromium resistance wire used in heating elements such as: electric toasters, hair dryers, electric ovens, heating blankets, etc. Nichrome wire becomes hot when an electric current is passed through the wire.

Radiative Heat Loss

The heat lost by radiation of electromagnetic waves (primarily infrared) from an object to the open sky.  All objects warmer than absolute zero will radiate heat. The movement of atoms (thermal energy) causes charged protons and electrons to vibrate, creating electromagnetic energy that radiates outward. 

Sight Tube

A simple collimation tube with a peep hole in one end that is used to center the secondary mirror in the telescope draw tube.

Astrophotography Terms


An automated system for tracking an object during astrophotography. A guidestar is selected and centered in a camera connected to a guide scope or off axis guider. A computer makes corrections to the telescope mount to keep the guide star at a constant position in the digital camera frame.

Barn Door Tracker

A simple tracking platform composed of a camera mounted to the top of two hinged boards separated by a threaded rod. The hinge is polar aligned and turning the threaded rod at a constant rate allows the camera to track celestial objects.

ccd Camera 

A ccd (charge couple device) camera uses a light sensitive chip to convert light into an electric signal; this technology is used in consumer digital cameras and camcorders.


The DSI (Deep Sky Imager) Pro is a monochrome ccd camera produced my Meade Instruments.

Field Derotator

A motor that compensates for field rotation by rotating an astronomical camera. The field derotator rotates the camera at the same rate as a celestial object rotates, negating field rotation. 

Field Rotation

The rotation of celestial objects during tracking with an Alt-Az mounted telescope. 

Guide Scope

A small telescope mounted onto the main telescope. The guide scope is orientated to point an illuminated crosshair (reticle) at a predetermined star during imaging. The stars location in the guidescope is periodically checked and the main telescope is repositioned to keep the guidescope on the same star. This (guiding) corrects for small drive errors and keeps the image correctly centered in the ccd camera during imaging.

Guidescope Rail

A guidescope rail is a metal plate that attaches the guidescope to the optical tube assembly. The guidescope is usually mounted inside guidescope rings, that attach to the guidescope rail. Spacers called radius blocks may be mounted between the optical tube and guidescope rail.


Making small adjustments to keep the telescope centered on a celestial object during astrophotography. Guiding corrects for the small errors that all telescope mounts and polar alignments contain.

Illuminated Reticle Eyepiece

A eyepiece with an illuminated reticle (crosshair) that is used with a guide scope. These eyepieces are also used in aligning equatorial mounts to the celestial pole.

Radius Blocks

Spacers used to attach a guidescope rail to a curved telescope tube. The radius block typically has a flat side that attaches to the guidescope rail and a curved side that fits the optical tube curvature.

RGB Filter Set

A set of red, green, and blue filters that are used with a monochrome ccd camera to produce a color image. Multiple pictures (monochrome + one picture through each color filter) are acquired and digitally merged to form a color photo.  Since many monochrome ccd cameras are more sensitive than color ccd cameras, there are sensitivity advantages to this type system.

Drive and Gear Terms


A cumulative error in gear systems occurring when the gears change direction. Backlash occurs because when the gears change direction, the teeth must move a small distance before they engage one another. Over time, backlash error accumulates and can affect GoTo pointing accuracy.

Band Clamp Gear

A worm wheel formed by wrapping a band clamp around a cylinder or disk.

Drive Corrector

A frequency transformer used to vary the speed of synchronous telescope drive motors. Many drive correctors included an invertor, allowing operation from either AC current or a 12 V car battery. Varying the drive corrector setting changed the rate at which an oscillating switch vibrated,  produced a square wave (mains electric is a sine wave). Adjusting the drive corrector speed setting changed how fast the switch oscillated, changing the electric frequency, and speeding up or slowing down the synchronous drive motors.

Friction Drive

A drive system that transfers rotational motion by frictional contact between multiple wheels.

Gear Reduction

Using a series of different sized gears, shafts, wheels, or other components to reduce the rate of rotation between elements in a drive train.


A rotating cutting tool used to cut the teeth in a worm wheel.

Hobbing Gears

Using a rotating hob to cut a worm wheel from a rotating metal or plastic gear blank.

Moulded Gear

A gear produced by pressing a threaded rod into a wheel with a center groove filled with resin or polymer. The resin or polymer hardens to produce molded gear teeth.

Shaft Coupler

A device that couples two rotating shafts of equal or different diameter and transfers torque.

Spur Gear

A gear with teeth around it's circumference that meshes with similar gearing on another device and transfers rotation.

Synchron Motor

Synchron® motors are the brand of AC synchronous motors found in the 1970's to early 1980's orange tube Celestron telescope drive bases. Dual 1 RPH Synchron® motors were used in an attempt to minimize backlash error.

Synchronous Motor

An AC motor used in 1970's to early 1980's telescope drives, clocks, record turntables, etc. Synchronous motors use AC current to produce a rotating magnetic field. The electrical frequency determines the rate at which the magnetic field rotates inside the motor, so motor speed is dependent upon electric frequency. Adjustment of synchronous motor speed required a device called a drive corrector.

Threaded Rod Gear

A worm wheel formed by wrapping a threaded rod around the circumference of a circle.


A gear formed in the shape of a screw that transfers rotation to a worm wheel.

Worm Drive

A drive system consisting of a worm transferring rotation to a worm wheel, where both drive axis are at 90 degrees relative to each other.

Worm Wheel

A gear with the same thread as a matching worm.

GoTo System Terms 


A computerized system employing motors, drive gears, and computer programming where an object is selected from a database and the telescope will automatically GoTo the selected object.

Meade 492 Motor Kit

A system manufactured by Mead Instruments that includes two drive motors, 60 tooth worm wheels, matching worms, mounting assemblies, computer interface board, and simple hand controller. This system, supplied with the simple hand controller, allows manual control of the telescope drive motors. A version 495 or 497 Autostar is required for full GoTo control.

Meade Autostar

A system manufactured by Mead Instruments for automated telescope positioning and tracking of celestial objects. The Autostar hand controller contains necessary programming to construct a mathematical model relating telescope position to celestial coordinates. The latest version (497 Autostar controller) can be updated via the www.

Observatory Terms 

Arch Support

Small wood blocks that connect the steel drywall frame arches to the dome ring.

Base Ring

A three layer plywood ring that sits on top of the base structure. The dome ring sits on top of bearings attached to the observatory base structure or base ring (depending upon the design).

Base Structure

The observatory structure that supports the dome at the same height as the telescope mount altitude bearings. The dome ring sits on top of bearings attached to the observatory base structure or base ring (depending upon the design).

Birdcage Dome

An observatory dome frame consisting of two central arches and usually 10 peripheral arches. The arches are usually constructed from 2 or 3 layers of thick plywood. This type of dome frame resembles a birdcage.


The left over tar fraction from distillation of crude oil. Bitumen is used for paving roads and in waterproofing materials.

Bituwell Plates

Bitumen impregnated sheets of corrugated inorganic-organic fibers, mainly used in Europe for carport and shed roofs. Bituwell plates are very strong yet lightweight (5.4 kg per 0.93 m x 2 m x 3 mm sheet).  

Central Arch

Two birdcage dome structural elements that form the observatory dome slit. The two central arches are chords across the base ring, and are less than the base ring diameter.

Dome Centering Bearings

Bearings contacting the inside and top surfaces of the dome ring, preventing the dome from moving side-to-side or lifting off of the base structure as it's rotated.

Dome Bearing

Bearings mounted on top of the base structure or base ring. The dome ring rotates on top of the dome bearings.

Domed Observatory

An observatory consisting of a rotating dome placed on top of a base structure. These observatories are more difficult to build than roll off observatories, but give better wind protection.

Dome Ring

The mulitlayered plywood ring that supports the dome. The central and peripheral arches attach to the top of the dome ring. The dome ring underside rotates on bearings attached to the observatory base structure or base ring (depending upon the design).

Dome Slit Cover

A removable door that covers the dome slit opening

Dome Slit Opening

The opening between the central arches allowing light into the telescope.

Foam Geodesic Dome

An observatory dome design consisting of a geodesic structure of styrofoam triangles (or other shapes), covered with fiberglass or some other structural, weather resistant material.

Peripheral Arch

Birdcage dome structural elements that start on the dome ring, extend toward the dome ring center, and connect with the central arches. There are usually 5 peripheral arches equally spaced along each central arch. 


The structure that supports the telescope mount. The pier extends down through the observatory floor without making contact with the floor or observatory structure; this reduces vibrations to the telescope from dome rotation.

Rain Skirts

Exterior sheets or strips that cover the gap between the observatory dome ring and the base structure, reducing drafts and makes the observatory weathertight.

Roll Off Observatory

An observatory consisting of a simple building with a roof that rolls off on tracks. These observatories are generally easier to build than domed observatories, but give less wind protection than domes.

Steel Drywall Supports

3 sided bars of 1 mm sheet steel, used in the construction of non-load bearing interior walls. These supports often contain pressed channels and/or folded lips that can greatly increase their strength without adding significant weight.

Telescope Terms

Achromatic Lens

A lens consisting of two different lenses (crown glass and flint glass) that converges red and blue light to the same focal point; other wavelenghts (green) will still give some degree of chromatic aberration.  


The diameter of an astronomical mirror or lens

Apochromatic Lens

A lens consisting of two or more different optical materials, which converge red, blue, and green light to the same focal point.

Bahtinov Focusing Mask

A focusing device for astrophotography invented by Pavel Bahtinov, consisting of a plate with three sections of parallel slits that fits over the end of the telescope. Each section of parallel slits produces a different diffraction spike for each bright object: a long cross composed of two diffraction spikes centered on the bright object and a third diffraction spike within the cross.  When the center diffraction spike is symmetrically placed within the cross formed by the other two diffraction spikes, then the object is in focus.


An 8 inch aperature  ƒ10 Schmidt-Cassegrain telescope (SCT) manufactured by Celestron International. The C8 was introduced in 1970 and was the first high volume-low cost SCT.


A 90mm Maksutov-Cassegrain telescope manufactured by Celestron International.

Catadioptic Telescope

A telescope type containing both lenses and mirrors. Catadioptic telescope designs offer advantages over reflector telescopes due to shorter tube length and reduced optical aberrations.

Chromatic Aberration

Chromatic aberration is when different frequencies (colors) of light converge at different focal lengths and is caused by a len's refractive index deecreasing as the light wavelength increases.


Adjusting the orientation of the primary and secondary mirrors to give the best possible image.

Collimating Screws

Screws on the secondary mirror holder that adjust the secondary mirror's orientation (alignment) with respect to the primary mirror.

Comatic Aberration

Comatic aberration (coma) is a property of parabolic mirrors where point sources of light are focussed at different places depending on their distance from the mirror's central axis. This causes stars in the center of the mirror's field to be in focus where stars become more out of focus toward the edge of the mirror's field.

Comatic Circle

A circle formed in the optical plane when an off axis point of light strikes a lens or mirror with comatic aberration.  Overlapping comatic circles form a comet or cone like image.


Curving inward.


Curving outward.

Corrector Plate

A lens placed in the entrance to a telescope with a spherical primary mirror to correct for comatic aberration.

Diffraction Spikes

Spikes or points formed on star images, arising from diffraction of light around the secondary mirror spider vane assembly.

Dobsonian Telescope

A design attributed to John Dobson, which is a large Newtonian telescope on a very inexpensive but sturdy Alt-Az type mount.

Draw Tube

A tube that holds the eyepiece and can be rasied or lowered relative to the secondary mirror. Most draw tubes are 0.96, 1.25, or 2 inch. inner diameter.


The lens that magnifies the image formed at the telescope focal point. 

Focal Length

The distance from the surface of a parabolic telescope mirror to where the image is formed. Note that for a thin lens the definition is slightly different.

Focal Plane

An object located an infinite distance from an optical system forms an image on the rear focal plane. If the object is not at infinity, the image is formed at a different location.

Focal Point

Convex lenses and concave mirrors focus light to converge at a focal point that lies at the focal length from the lens or mirror.

Light Baffle

SCT light baffes are tubes placed in front of the primary mirror and around the secondary mirror that reduce the stray light inside a SCT optical system. 


The eyepiece enlarges the image formed at telescope focal point. The magnification equals the telescope mirror or lens focal length divided by the eyepiece focal length.

Maksutov Telescope

A telescope type invented in 1941 by Dimitri Maksutov. The Maksutov (MAK) telescope is a catadioptic telescope containing a spherical primary mirror, a spherical secondary mirror, and a spherical meniscus corrector plate.

Meniscus Corrector Plate

A spherical meniscus shaped corrector plate, found on Maksutov-Cassegrain telescopes. The corrector plate is located on the entrance to a telescope with a spherical primary mirror to correct for comatic aberration. 

Mirror Box

The truss tube telescope sections containing the mirror cells and mirrors, connected together with truss tubes.

Mirror Spot

A small secondary mirror attached directly to the back of the corrector plate in a Maksutov telescope.

Newtonian Telescope

A reflector telescope invented by Isaac Newton that consists of a parabolic primary mirror and a flat secondary mirror.  

Objective Lens

The large front lens in a refractor telescope, that gathers light and focuses it to the focal point.


Optical Tube Assembly. Just another term for a telescope.

Parabolic Mirror

A mirror with a parabolic cross section that focuses light to a common focal point.

Porro Prism

A device containing two right triangular glass blocks that uses total internal reflection to reinvert an upside down terrestrial image.

Primary Mirror

The large mirror in a telescope that gathers light and focuses it to an image.

Primary Mirror Cell

A device that holds the primary mirror and has adjustment screws to change the mirrors orientation (collimation).

Reflector Telescope

A telescope which uses mirrors to collect and focus light.

Refractor Telescope

A telescope which uses lenses to collect and focus light.


The Schmidt-Cassegrain telescope (SCT) consists of a spherical primary mirror, convex secondary mirror, and a Schmidt corrector plate (thicker in the middle and edge) to correct spherical aberration. 

Schmidt Corrector Plate

The Schmidt corrector plate is a lens that corrects incomming light so that light striking the outer portion of a spherical primary mirror focuses onto the same spot as light striking the inner portion. The Schmidt corrector plate is thicker at the edges and center., and flat on the side facing the primary spherical mirror.

Secondary Mirror

The small mirror that reflects the light gathered by the primary mirror out of the optical tube assembly and into the eyepiece.

Secondary Mirror Cell

A device that holds the secondary mirror and has adjustment screws to change the mirrors orientation (collimation).

Sector Box

A box that  connects the two sectors (altitude axis bearings) to the OTA.

Spherical Aberration

Spherical aberration is a property of spherical mirrors where light striking the mirror's center is focused farther from the mirror than light striking near the edge.

Spherical Mirror

A mirror with a circular cross section that focuses light to different focal points depending on where the light strikes the mirror. Light striking the mirror's center is focused farther from the mirror than light striking near the edge (spherical aberration).


A device consisting of vanes or tensioned wires that holds the secondary cell centered in the OTA.

Telrad Zero Pointing Device

A positioning or finder system that does not magnify or invert the image. The Telrad uses a system similar to a military heads up display to project illuminated circles onto a clear plate, through which the sky is viewed.

Truss Tube

A series of tubes forming a cage assembly that holds the primary and secondary mirrors.

Truss Tube Connector

A device that connects the truss tubes to the telescope mirror boxes.

Telescope Mount and Celestial Motion Terms


The rotational axis that elevates or depresses a telescope (up-and-down direction)


A telescope mount that allows the telescope to move up-and-down and side-to-side. The type of mounts typically found in toy telescopes.

Aristarcus of Samos

Aristarcus of Samos (310 BC- 230 BC) was a Greek astronomer and mathematician who was the first to propose that the sun was at the center of the solar system.


The rotational axis that moves the telescope parallel to the ground (side-to-side direction)


A coupling device allowing two components to rotate relative to each other.

Celestial Equator

The intersection of the plane containing the Earth's equator with the celestial sphere. Declination coordinates are measured in degrees above or below the celestial equator.

Celestial Sphere

An imaginary non-rotating sphere, centered on the Earth and its axis, that contains all celestial objects. As the Earth rotates within the celestial sphere (W. to E.), an observer on the Earth will see celestial objects move across the sky on the inside of this sphere (E. to W.).

Circumpolar Star

A circumpolar star is a star located close enough to the celestial pole that it never sets. The observers latitude determines how close to the celestial pole a star must be in order to be circumpolar.

Claudius Ptolemy

Claudius Ptolemy (90 AD-168 AD) was a mathematician, astronomer, and geographer who created the Ptolemaic model of planetary motion.

Cross Axis Mount

A telescope mount with the right ascension axis supported on both ends. A declination axis, holding the telescope and counterweight, attaches in the middle of the right ascension axis. This is a very solid, but unportable mount, and is best suited to fixed locations and observatories.

OTA Centering Bearings

A bearing placed on each side the telescope mount that keeps the telescope centered in the altitude axis. This prevents the telescope from moving side-to-side as it's elevated or depressed.


Coordinate lines that run parallel to the celestial sphere's equator, and are similar to latitude lines on Earth.

Declination Axis

The equatorial telescope mount axis that is perpendicular cross axisthe Earth's axis and moves the telescope parallel to right ascension lines on the celestial sphere.


The large circles that contain the epicycles in the Ptolemaic model of planetary motion. The deferents rotate around the Earth and move the epicycles through the zodiac.

Dobsonian Bearings

Simple mechanical bearings that use Teflon sliding against metal, plastic, flooring tile, or some other low friction component.

Dobsonian Mount

An Alt-Az type mount constructed traditionally from a 3 sided plywood box that rotates on a ground board. The telescope sits within a telescope box, which rests on top of the 3 sided plywood box.


The path the the sun, planets, and asteroids follow on the celestial sphere.

Ecliptic Plane

The plane containing the ecliptic.


The small planet containing circles that attach to the deferents in the Ptolemaic model of planetary motion. The epicycles rotate on the deferent, causing retrograde planetary motion.

Equatorial Fork Mount

An equatorial telescope mount consisting of a telescope suspended between two large forks with an inclined azimuth axis parallel with the Earth's polar axis.  The simplest configuration is to place an Alt-Az fork mount on an inclined wedge.

Equatorial Mount

A telescope mount where one axis is tilted parallel to the Earth's rotational axis. This type of mount is used for precise celestial tracking and astrophotography.

Equinoctial Points

The two points where the ecliptic crosses the celestial equator during the equinoxes.


The two times each year (March and September) when the sun crosses the celestial equator. During the equinoxes, night and day are of equal length.

First Point of Aries

The point where the path of the sun crosses the celestial equator during the March equinox. The First Point of Aries is the defined as the starting point for measuring right ascension (RA=zero).

Fork Mount

An Alt-Az type mount where the telescope is suspended between two large forks. 

German Equatorial Mount (GEM)

An equatorial mount with the telescope mounted to pivot around a right ascension axis that is parallel to the Earth's axis. The declination axis is perpendicular to the right ascension axis. The telescope is mounted off center along the declination axis, and requires a counterweight.

Heliocentric Model

A model of planetary motion where the sun is at the center of the solar system and the planes revolve around the sun. The idea of a sun centered solar system was first proposed in the 3rd century BC by Aristarcus of Samos. 

Hemispherical Mount

A telescope mount consisting of a telescope attached to a large sphere. The sphere is supported in a cup or similar device that allows it to rotate in all directions. Some telescope builders have used old bowling balls for the hemisphere.

March Equinox

The equinox occuring during March when the sun crosses the celestial equator at the First Point of Aries (defined as the starting point for measuring right ascension, RA=zero).

Nicolaus Copernicus

Nicolaus Copernicus (1473 AD-1543 AD) was a Polish astronomer who rediscovered the idea that the sun is at the center of the solar system and the Earth and other planets revolve around the sun. The idea of a sun centered solar sysytem was first proposed in the 3rd century BC by Aristarcus of Samos. 


The wobble in the Earth's axis that causes the Earth's pole to rotate in a 26,000 year cycle. Since celestial coordinates are defined relative to Earth's orientation in the celestial sphere, precession causes celestial coordinates change over the 26,000 year cycle.

Ptolemaic Model

An Earth centered model of planetary motion where planets are attached to rotating circles or spheres called epicycles. The epicycles are attached to a larger circle called a deterent that rotates around the Earth. This incorrect model gave an explanation of retrograde planetary motion and was widely accepted from the 2nd century AD until the 16th centrury AD.

Retrograde Motion

The apparent backwards motion of a planet against the stars when the Earth passes the planet in its orbit.

Right Ascension

Coordinate lines that run pole to pole on the celestial sphere, and are similar to longitude lines on Earth.

Right Ascension Axis

The equatorial telescope mount axis that is parallel to the Earth's axis and moves the telescope parallel to declination lines on the celestial sphere.

Roller Bearings 

Mechanical bearings made from casters, rollers, roller-skate or roller-blade wheels, etc.


Large circles that a telescope drive axis rotates around. The diameter through the sector is perpendicular to the axis of rotation and sectors can function as bearing or drive components.

Split Ring Mount

An equatorial mount that suspends the telescope inside a horseshoe shaped ring. The telescope rotates on a right ascension axis normal to the split ring face and parallel to the Earth's axis. 


An inclined plane that tilts an Alt-Az mount so that it functions as an equatorial mount.

Yoke Mount

An equatorial mount that suspends the telescope inside an inclined fork. The inclined fork is supported at both ends and forms a right ascension axis parallel to the Earth's axis. 


The point on the celestial sphere directly overhead of an observer.


The band of 12 constellations that contain the ecliptic, through which the sun and planets appear to move.